Ability to install Cluster, and updated to 5.5 by default

This installs Percona-XtraDB-Cluster-* if your node is server => true and cluster => true.

Complains (fails) if cluster is enabled without server.  I can probably approach this better.

Got rid of the dependencies on the 'common' packages, which will be automatically installed by the package manager anyway.

Also switched the default to 5.5 to make behavior more predictable, since the Cluster packages are all Percona 5.5.  You can manually merge and remove that if you like.

This hasn't yet added other cluster features (notably, won't actually configure a cluster yet).